While the Picture Gorge Basalt was flooding most of the John Day basin, several volcanoes in the vicinity of Strawberry Mountain erupted andesitic to rhyolitic lava and ash, and built up cones similar to Mt. Hood and the other high peaks of the Oregon Cascades. Geologists have named the remnants of these cones the Strawberry Volcanics. Because these volcanoes rose several thousand feet above the top of the Picture Gorge Basalt, ash and erosional debris from them washed out over the basalt; these materials constitute the Mascall Formation. Although the mountains were timbered, the lowlands probably were open and grassy. Bones and teeth of a pony-size three-toed horse, _Merychippus_, have been dug from the Mascall Formation. When the eruptions ceased in Early Pliocene time (about 10 million years ago), the entire Blue Mountain region probably resembled the eastern part of the Cascade Mountains in Oregon today, particularly the area between Mt. Hood and Crater Lake. The present Strawberry-Aldrich Mountain range and the ridges and valleys north of it were formed when the Earth’s crust buckled and broke under strong compressive forces from the north and south. Partly by bending or folding, and partly by breaking along the John Day and other faults, the Strawberry-Aldrich Mountains were gradually raised a mile and a half to two miles above the valley to the north. The floor of the main John Day River valley was filled with gravels eroded from the rising mountains and, as the folding and faulting and erosion slowed, an extensive, broad, gently sloping surface was formed on top of the valley fill. While these gravels (called the Rattlesnake Formation) were accumulating, however, a great volcanic eruption spread an ash flow more than 100 feet thick over the entire length of the John Day valley floor; remnants of this flow form prominent rim-rocks between the town of John Day and Picture Gorge. Bones of _Hipparion_, a horse the size of a pony which had feet and teeth like modern horses, have been found in the gravels under the ash flow. _THE PRESENT LANDSCAPE_ Differences in the rates of erosion of various rocks were as important as folding and faulting in forming the topographic features in the John Day Country. The chief agents of erosion are chemical weathering processes that eventually break down or decompose all rocks, and running water which carries away the weathered or partly weathered and broken material. Picture Gorge is narrow because the basalts in the walls are very resistant to weathering and break along vertical joints into large blocks that do not move easily. In contrast, the John Day River has cut a broad, flat-floored valley in the Mascall Formation at the south end of Picture Gorge because the ashy beds weather to fine clay which is easily washed away.